Minor cleanup of JIT exception handling code.
Replace instances of __attribute__((hot, fastcall)) with a macro.

Additionally, only include fastcall when compiling on __i386__, since it gets
ignored on other platforms.
Remove some old code that was testing the error messages produced with matrices
whose total size exceeded 2^31.  Now that R can use 64 bit objects, it just
causes massive allocations, which isn't useful.